Abstract
The nonlinear wave modulation of planar and nonplanar (cylindrical and spherical) ion-acoustic envelope solitons in a collisionless unmagnetized electron-positron-ion plasma with two-electron temperature distributions has been studied. The reductive perturbative technique is used to obtain a modified nonlinear Schrodinger equation, which includes a damping term that accounts for the geometrical effect. The critical wave number threshold K(c), which indicates where the modulational instability sets in, has been determined for various regimes. It is found that an increase in the positron concentration (alpha) leads to a decrease in the critical wave number (K(c)) until alpha approaches certain value alpha(c) (critical positron concentration), then further increase in alpha beyond alpha(c) increases the value of K(c). Also, it is found that there is a modulation instability period for the cylindrical and spherical wave modulation, which does not exist in the one-dimensional case.